I was thinking the dimmer might be the issue as well. I don't know the physics of light bulbs, but always being on a dimmer might cause more wear on them than normal use. If you have to always dim a 100w bulb, try going down to a 75w bulb without a dimmer and see if that lasts longer. (Or whichever wattage it takes to maintain the temp you're aiming for)

As far as where you buy it from.....a Zoomed bulb from Petco and a Zoomed bulb from RBI are still both the exact same bulb. I really don't think the source makes a difference.

Switching to halogen might help...but make sure you're getting the kind of heat output you need for a beardie.